The present invention is directed to a device for hooking flexible printing plates on a printing cylinder located in a rotary printing machine.
Rotary printing machines are usually called flexographic printing machines and are often used in paperboard industry. In these machines, flexible printing plates are fitted on a cylinder which rotates so as to transfer the ink onto a surface to be printed. In these machines, sheets of cardboard are taken from the top of a pile and carried individually and successively through the printing station and, eventually, through a die-cutting station and then, finally, are gathered in a delivery station.
With this kind of machine, the action of exchanging printing plates on the printing cylinder is usually troublesome, specifically the action of fitting the printing plate which is to be executed quickly in order not to unnecessarily delay the machine during the change of a sheet size or a printing size. A printing plate usually includes one or several rubber or plastic printing forms, which are fitted on a flexible support sheet consisting either of a strong paper or of a rubber or plastic material, so that they may be applied and wrapped around a printing cylinder.
One of the first means known for hooking a printing plate on a cylinder consists in clamping the sheet on a wooden piece which belongs to the cylinder. However, with long runs, these clamps have the tendency of damaging the supporting sheet as well as the wooden piece which has to be replaced regularly. Moreover, if the printing plate is in the wrong position, it is necessary to completely take it off the cylinder and then reposition it on the cylinder, which may double or even triple the downtime of the machine.
Another hooking device includes a rigid strip at both the upstream and downstream ends of the printing plate. The upstream strip is engaged in a fixed groove of the cylinder, whereas the downstream strip engages in a jointed or adjustable groove in such a way that the printing plate can be placed under tension. However, the dimension of the printing plate has to be rather accurate and problems about the means for setting the adjustable groove into operation can occur.
In fact, the real problem consists in keeping the whole of the printing plate applied on the cylinder, which does not necessarily involve a strong tensioning of the printing plate.
French Patent Documents FR 2,434,035 and FR 2,196,910 each reveal plate-supporting cylinders whose outer cylindrical surface is provided with a series of grooves connected through a radial aperture to an inner chamber of the cylinder in which a vacuum can be created by means of a vacuum pump. It is possible to arrange either a series of parallel circular grooves, or a groove meshing, or simply one groove running helicoidally along the cylinder. When these grooves are covered by the printing plate, tight spaces are created, which, once a vacuum is created in this space, will hold the printing plate on the cylinder.
The hooking device described in French Patent Document 2,196,910 includes two parallel grooves arranged side-by-side and made almost tangentially along a generatrix of the cylinder. The grooves open on their opposite sides so that one of them is destined to receive the front edge of the printing plate and the other receives the downstream edge. These printing plate edges are held in position by means of bolts which extend radial across the grooves and engage in apertures threaded in the cylinder. However, this device is defective by the fact that the edges are positioned or held in position at only two or three points. Moreover, the insertion of the edges in the grooves can be achieved more or less easily depending on the thickness or the supporting sheet.
The hooking device described in French Patent Document FR 2,434,035 is based on the existence of fixing pawns or pins protruding from the cylinder and on which openings of the plate are engaged. The openings are arranged a little away from the front and downstream edge of the printing plate. A board is then inserted between these pins in order to keep the edges applied on the cylinder. Again, the position accuracy of the front edge cannot be guaranteed by the sole holding at these points.
The most common device, which is called a Matthews device, includes a hooking strip with a U-shaped cross section, which is mounted at one end on the printing plate. This strip is hooked in a corresponding groove of a printing cylinder having a U-shaped cross section. A series of elastic tensioners are fitted on the other end of the printing plate. Every tensioner consists of a U-shaped hook which can engage in another groove of the cylinder, which groove also has a U-shaped cross section. These elastic tensioners allow a strong tensioning of the printing plate on the cylinder.
However, since the fitting of every tensioner may be fastidious, it is also possible to replace these tensioners by a large adhesive strip, which fixes directly the downstream edge of the printing plate to the cylinder.
Even though the device functions satisfactorily, there still remains the risk that the fixtures of the downstream edge undergo release of the tension, which release may further progress until the printing plate falls between the inking cylinders or travels through the rest of machine, if the latter is still in operation. While some devices for mechanical tensioning or magnetic fastening of the printing plate diminishes this risk, they have the drawback of creating a complicated setting operation and of a complex adjustment of the ultimate position for the plate.